What is a procedure where participants are assigned to groups based on random selection but later treated equally?

The procedure described in the question involves assigning participants to groups through random selection and ensuring that they are treated equally throughout the study. This aligns with the definition of a randomized control trial, which is a type of experimental design where participants are randomly assigned to either a treatment group or a control group. This randomization helps to eliminate bias and ensure that the groups are comparably balanced regarding known and unknown confounding variables, allowing for more valid conclusions about the effects of the treatment.

In a randomized control trial, participants receive the same conditions in terms of treatment aside from the independent variable being tested, making the methodology rigorously controlled. This treatment equality ensures that differences in outcomes can be attributed directly to the manipulation of the independent variable rather than other extraneous factors.

While other study designs like control group designs or quasi-experimental designs involve grouping participants, they may not incorporate randomization or equality of treatment in the same structured manner as randomized control trials do, which can impact the reliability of results derived from those approaches. Therefore, the focus on random selection and equal treatment in this context confirms that the correct answer is the randomized control trial.